And why are more men affected than women?

Below:

May 27, 1999 — Both men and women can lose hair for a variety of reasons, including poor nutrition, thyroid disease, hormonal fluctuations, genetics and age. But by far, the most common type of hair loss is the one that affects many men in the prime of their lives. It’s known as androgenetic alopecia or, more commonly, pattern baldness. And while ancient Greek doctors made an initial stab at explaining what causes pattern baldness, it wasn’t understood until 1949.

Anatomist James B. Hamilton, through some simple observations and experiments conducted during the middle of the century, showed that male pattern baldness was dependent on the complex interaction of three factors: androgens, genetics and age. Androgens are hormones whose primary role is inducing and maintaining male secondary sex characteristics. The principal androgen, testosterone, is produced in the testes and adrenal glands of men and, in much smaller quantities, in the ovaries and adrenal glands of women.

Testosterone plays a key role in male pattern baldness, but it only affects hair follicle cells that are genetically predisposed to the condition. In susceptible men, testosterone comes into contact with an enzyme found in the hair follicles. When this occurs, the testosterone is converted to dihydrotestosterone (DHT), a more potent androgen that has the ability to bind to receptors in follicles, explains Dr. Richard A. Strick, a dermatologist at the University of California at Los Angeles. This binding, in turn, can trigger a change in the genetic activity of the cells, which initiates the gradual process of hair loss.

“Pattern baldness can be genetically inherited from either the maternal or the paternal side of the family,” says Strick. What’s inherited, he explains, is the tendency for certain hair follicles, in the presence of DHT, to become progressively smaller over time. This causes the growing cycle of the hair follicle to shorten, more hairs to be shed, and the existing hair to become thinner and thinner. Some follicles eventually die, while others shrink to a very small size incapable of sustaining healthy hair.

While the presence of androgens is critical for the genetic predisposition to be expressed, age is equally influential. The biological mechanisms of this are not entirely understood, but we do know that the susceptibility of pattern baldness increases with age. Different ethnic groups also have varying susceptibility levels to pattern baldness: Chinese, blacks and Native Americans are more likely to keep a full head of hair than whites. Statistics show that roughly 30 percent of white men in their 30s are affected by pattern baldness, as are 40 percent in their 40s, and so on to the point that 80 percent of white men are affected by the time they are 80 or older.

Why are more men bald?

Although the mechanisms of pattern baldness are similar in men and women, there are some significant differences. The most notable one is that pattern baldness is more prevalent in men than in women. According to Strick, this fact can be largely attributed to the higher levels of testosterone in men and the key role that the hormone plays in hair loss.

Another difference is that the actual pattern of baldness varies with gender. Women usually retain their frontal hairline and lose the hair over the top of the scalp. Men, on the other hand, usually experience a receding hairline that leaves a horseshoe-like rim of hair around the back and sides of the head. Strick and other researchers suspect that this difference can be mainly attributed to the genetic makeup of hair follicles in different parts of the scalp, and therefore the concentrations of DHT, enzymes and androgen receptor sites in each.

Some research has also shown that additional hormonal factors may come into play where female hair loss is concerned. This is supported by the fact that many women lose some hair during menopause, when significant hormonal changes are taking place.

Do you have “Cosmic Questions” about the universe’s great mysteries — such as the prospects for interstellar travel? Or are you wondering about “Common Curiosities,” such as the reasons we snore and what we can do about it? In either case, e-mail your suggestions to mysteries@msnbc.com, or discuss your questions and answers on the Mysteries of the Universe Bulletin Board. Your topic could become the subject of a future installment of “Mysteries of the Universe.”